This relates generally to imaging systems, and more particularly to imaging systems with improved capacity.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) may be formed from a two-dimensional array of image sensing pixels. Each pixel receives incident photons (light) and converts the photons into electrical signals. Image sensors are sometimes designed to provide images to electronic devices using a Joint Photographic Experts Group (JPEG) format.
Image pixels include photodiodes that receive light and converts photons into electrical signals. A photodiode typically includes a p-n junction formed within a silicon substrate. At the p-n junction (i.e., between p-type and n-type regions) a depletion region forms and establishes an electric field. When photons are received by the substrate, holes and electrons may be generated. The holes and/or electrons can provide current flow through the p-n junction due to the electric field established by the depletion region. The electrical potential associated with the electric field varies across the p-n junction and has a dome-like shape when plotted along the length of the p-n junction. In other words, the electrical potential may reach a peak at the center across the length of the p-n junction boundary, whereas regions at the edges of the p-n junction may have somewhat lower electrical potential. The electrical potential at a given location across the length of the p-n junction is related to the amount of charge that is stored across the depletion region at that location, and therefore electrical potential profiles exhibiting dome-like shapes have unequal charge distribution along the p-n junction.
In some scenarios, a counter-doping implant is provided within the n-type region of the p-n junction to flatten the dome-like shape of the electrical potential across the length of the p-n junction. The counter-doping implant is a p-type region (i.e., doped with p-type dopants) that is fully contained within the n-type region and has a lower concentration than the n-type region to ensure that the entirety of the n-type region remains n-type (i.e., the concentration of n-type dopants within the counter-doping implant is greater than the concentration of p-type dopants).
Photodiode characteristics and performance can be constrained by fabrication process limitations. With ever-decreasing process dimensions and photodiode sizes, the amount of area that is available for the p-n junction of a photodiode is also reduced. With reductions in the amount of available p-n junction area, the pixel well capacity (i.e., the capacity of the pixel to store charge) tends to be reduced.